> AspectJ is a language, these aspects are visible and have their own language abstractions, just separate from the application code. Just like you have monad implementations separate from their uses. So writing aspects is also "plain code".
Code containing aspects is certainly not plain Java code, because the aspect annotations (or, worse, invisible string-name based pointcuts) change the behaviour of the code they're on, breaking the normal rules of the language. You'll see "impossible" behaviour at runtime: call a method and the call stack jumps to a completely different method. A method throws an exception that's thrown nowhere in its body. If you want to say that AspectJ is a language in its own right, then it's a language that breaks all the rules of good language design: it essentially contains COMEFROM as a core language feature. Whereas monad implementations are (generally) plain old code that follows the normal rules of the language.
> Also, AspectJ isn't Java
It (or other libraries that do much the same thing) is the vast majority of Java as it actually exists in the real world. I've yet to see a substantial Java codebase that wasn't using some form of AOP or reflection (even if only indirectly via these big frameworks).